1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
There is known an image forming apparatus in which four photosensitive drums are used for each of four colors, in order to transfer overlappingly four toner images each comprising each of four colors onto a recording medium and thereby form a color image on the recording medium. In this type of image forming apparatus, there is known a technique to prevent a misregistration (color drift) among toner images by forming calibration patterns for respective colors on a carry belt while each photosensitive drum rotates one round and detecting each of the formed calibration patterns to calibrate the misregistration.
Japanese Patent Application Laid-open No. H11-65208 discloses a technique to improve the accuracy in detecting the misregistration by forming a plurality of toner marks (patterns) and detecting the plurality of toner marks in a color image forming apparatus.
Recently, there is a need for an image forming apparatus in which five or more photosensitive drums are used for each of multiple colors (five or more) so that toner images each comprising each of multiple colors are transferred overlappingly onto a surface of the recording medium, in order to obtain a high quality image. In this type of multi-color image forming apparatus, the number of calibration patterns formed on the carry belt while each photosensitive drum rotates one round increases, since calibration patterns for each color are formed on the carry belt when calibrating the misregistration.
In the technique disclosed by Japanese Patent Application Laid-open No. H11-65208, since a plurality of toner marks are formed for each color, the number of toner marks increases in a case that a color image is formed by means of five or more photosensitive drums each corresponding to each of five or more colors, in comparison with a case that a color image is formed by means of four photosensitive drums each corresponding to each of four colors. Therefore, an interval between toner marks is shortened and the accuracy in detecting the toner mark may be lowered.